Various optical methods for the measurement of the relative velocity and/or motion of an object with respect to a measurement system exist. Each method and apparatus is characterized by the kinds of objects and the kinds of motions on which it operates.
The kind of measurable objects may be broadly divided into several groups, including:
A specially patterned object, for example, a scale. PA1 A reflecting surface, for example, a mirror. PA1 A small particle (or few particles), for example precursor particles or bubbles suspended in fluid. PA1 An optically contrasting surface, for example, a line or dot pattern. PA1 An optically diffuse object, for example, blank paper. PA1 Axial movement toward or away from the measuring device. PA1 Transverse (or tangential) motion, where the spacing between the measuring device and the object is essentially constant. PA1 Rotational motion, where the object orientation with respect to the measurement device is changing. PA1 A coherent light source illuminates the object the motion of which needs to be measured. PA1 The illuminated object (generally an opaque surface) consists of multiple scattering elements, each with its own reflection coefficient and phase shift relative to the other scattering elements. PA1 The individual reflection coefficients and phase shifts are substantially random. At a particular point in space, the electric field amplitude of the reflection from the object is the vector sum of the reflections from the illuminated scattering elements, with an additional phase component that depends on the distance between the point and each element. PA1 The light intensity at a point will be high when contributions generally add in phase and low when they generally add out of phase (i.e., subtract). PA1 On a planar surface (as opposed to a point), an image of random bright and dark areas is formed since the relative phase retardation of the source points depends on the location in the plane. This image is called a "speckle image," composed of bright and dark spots (distinct "speckles"). PA1 The typical "speckle" size (the typical average or mean distance for a significant change in intensity) depends primarily on the light wavelength, on the distance between the object and the speckle image plane and on the size of the illuminated area. PA1 If the object moves relative to the plane in which the speckle image is observed, the speckle image will move as well, at essentially the same transverse velocity. (The speckle image will also change since some scatterers leave the illuminated area and some enter it). PA1 The speckle image is passed through a structure comprising a series of alternating clear and opaque or reflecting lines such that the speckle image is modulated. This structure is generally a pure transmission grating, and, ideally is placed close to the detector for maximum contrast. PA1 The detector translates the intensity of the light that passes through the structure to an electrical signal which is a function of the intensity (commonly a linear function). PA1 When the object moves with respect to the measuring device, the speckle image is modulated by the structure such that the intensity of light that reach the detector is periodic. The period is proportional to the line spacing of the structure and inversely proportional to the relative velocity. PA1 By proper signal analysis, the oscillation frequency can be found, indicating the relative velocity between the object and the measurement device. PA1 illuminating the surface with incident illumination such that illumination is reflected from portions of the surface; PA1 placing a partially reflecting object, which is part of the measuring device, adjacent to the surface, wherein part of the incident illumination is reflected or diffracted by the object, as a reference illumination; PA1 coherently detecting the illumination reflected from the surface, utilizing the illumination reflected from or diffracted by the object as a local oscillator, to form a signal; PA1 determining the relative motion of the surface from the signal; PA1 varying the phase of the illumination reflected from or diffracted by the object with respect to that reflected from the surface; and PA1 determining the direction of relative motion parallel to the surface based on a characteristic of the signal caused by said varied relative phase. PA1 providing a transparent material, preferably a piezoelectric material, between the object and the surface; and PA1 electrifying the material such that its optical length in the direction of the illumination varies. PA1 placing a partially reflecting object, which is part of the measurement device, adjacent to the surface; PA1 illuminating the surface with incident illumination such that illumination is reflected from portions of the surface and such that illumination is reflected from or diffracted by the object; PA1 coherently detecting the illumination reflected from the surface utilizing the illumination reflected from or diffracted by the object as a local oscillator to form a signal; PA1 determining the relative motion of the surface perpendicular to and parallel to the surface from the signal. PA1 detecting amplitude or phase variations of the reflected illumination; and PA1 detecting a frequency shift of the reflected illumination; and PA1 determining the relative motion comprises: PA1 periodically moving the object in a direction perpendicular to its surface to add a periodic phase shift to the illumination reflected therefrom; and PA1 utilizing said phase shift to measure the motion of the surface. PA1 a housing having an aperture facing a surface; and PA1 an optical motion detector which views the surface through the aperture, wherein the optical motion detector utilizes the method of the invention to determine the translation of the housing with respect to the surface.
The kind of measurable motions may be broadly divided into several groups, including:
It is also useful to classify the measurement devices according to the number of simultaneously obtainable measurement directions (one, two or three dimensional) and the number of critical components (light sources, light detectors, lenses, etc.).
It should be noted that a specific method may be related to more than one group in the above classification schemes.
A number of systems capable of non-contact measurement of the transverse velocity and/or motion of objects using optical means have been reported. These methods can include Speckle Velocimetry methods and Laser Doppler Velocimetry methods. Other methods of interest for understanding the present invention are Image Velocimetry methods, homodyne/heterodyne Doppler Velocimetry or Interferometry methods and Optical Coherence Tomography (OCT).
Speckle Velocimetry methods are generally based on the following operational principles:
For these methods high accuracy frequency determination requires a large detector while high contrast in the signal requires a small detector. A paper by Popov & Veselov, entitled "Tangential Velocity Measurements of Diffuse Objects by Using Modulated Dynamic Speckle" (SPIE 0-8194-2264-9/96), gives a mathematical analysis of the accuracy of speckle velocimetry.
U.S. Pat. No. 3,432,237 to Flower, el. al. describes a speckle velocimetry measuring system in which either a transmission pattern or a pin hole is used to modulate the speckle image. When the pin-hole is used, the signal represents the passage of individual speckles across the pin hole.
U.S. Pat. No. 3,737,233 to Blau et. al. utilizes two detectors in an attempt to solve the problem of directional ambiguity which exists in many speckle velocimetric measurements. It describes a system having two detectors each with an associated transmission grating. One of the gratings is stationary with respect to its detector and the other moves with respect to its detector. Based on a comparison of the signals generated by the two detectors, the sign and magnitude of the velocity may be determined.
U.S. Pat. No. 3,856,403 to Maughmer, et al. also attempts to avoid the directional ambiguity by providing a moving grating. It provides a bias for the velocity measurement by moving the grating at a velocity higher then the maximum expected relative velocity between the surface and the velocimeter. The frequency shift reduces the effect of changes in the total light intensity (DC and low-frequency component), thus increasing the measurement dynamic range and accuracy.
PCT publication WO 86/06845 to Gardner, et al. describes a system designed to reduce the amplitude of DC and low frequency signal components of the detector signal by subtracting a reference sample of the light from the source from the speckle detector signal. The reference signal is proportional to the total light intensity on the detector, reducing or eliminating the influence of the total intensity variations on the measurement.
This reference signal is described as being generated by using a beam-splitter between the measured surface and the primary detector by using the grating that is used for the speckle detection also as a beam-splitter (using the transmitted light for the primary detector and the reflected light for the reference detector) or by using a second set of detectors to provide the reference signal. In one embodiment described in the publication the two signals have the same DC component and opposite AC components such that the difference signal not only substantially removes the DC (and near DC) components but also substantially increases the AC component.
In U.S. Pat. No. 4,794,384, Jackson describes a system in which a speckle pattern reflected from the measured surface is formed on a 2D CCD array. The surface translation in 2 dimensions is found using electronic correlation between successive images. He also describes an application of his device for use as a "padless optical mouse."
Image velocimetry methods measure the velocity of an image across the image plane. The image must include contrasting elements. A line pattern (much like a grating) space-modulate the image, and a light-sensitive detector is measuring the intensity of light that pass through the pattern. Thus, a velocity-to-frequency relation is formed between the image velocity and the detector AC component. Usually, the line pattern moves with respect to the detector so that the frequency is biased. Thus, the direction ambiguity is solved and the dynamic range expanded.
A paper by Li and Aruga, entitled "Velocity Sensing by Illumination with a Laser-Beam Pattern" (Applied Optics, 32, p.2320, 1993) describes image velocimetry where the object itself is illuminated by a periodic line structure (instead of passing its image through such a pattern). The line pattern is obtained by passing an expanded laser beam through periodic transmission grating (or line structure). According to the suggested method the object still needs to have contrasting features.
There exist a number of differences between Image Velocimetry (IV and Speckle Velocimetry (SV). In particular, in SV the random image is forced by the coherent light source, whereas in IV an image with proper contrasting elements is already assumed. Furthermore, in SV the tangential velocity of the object is measured, whereas in IV the angular velocity is measured (the image velocity in the image plane is proportional to the angular velocity of the line of sight).
In U.S. Pat. No. 3,511,150 to Whitney et. al., two-dimensional translating of line patterns creates a frequency shift. A single rotating circular line pattern creates all the necessary translating line patterns at specific elongated apertures in a circular mask. The frequency shift is measured on-line using an additional detector measuring a fixed image. The line pattern is divided to two regions, each one adapted for the measurement of different velocity range. The system is basically intended for image motion compensation in order to reduce the image blur in aerial photography. Also, it is useful for missile homing heads.
U.S. Pat. No. 2,772,479 to Doyle describes an image velocimetry system with a frequency offset derived from a grating on a rotating belt.
Laser Doppler Velocimeters generally utilize two laser beams formed by splitting a single source which interfere at a known position. A light-scattering object that passes through the interfering space scatters light from both beams to a detector. The detector signal includes an oscillating element with frequency that depends on the object velocity. The phenomena can be explained in two ways. One explanation is based on an interference pattern that is formed between the two beams. Thus, in that space the intensity changes periodically between bright and dark planes. An object passing through the planes scatters the light in proportion to the light intensity. Therefore, the detected light is modulated with frequency proportional to the object velocity component perpendicular to the interference planes. A second explanation considers that an object passing through the space in which both light beams exist, scatters light from both. Each reflection is shifted in frequency due to the Doppler effect. However, the Doppler shift of the two beams is different because of the different angles of the incident beams. The two reflections interfere on the detector, such that a beat signal is established, with frequency equal to the difference in the Doppler shift. This difference is thus proportional to the object velocity component perpendicular to the interference planes.
It is common to add a frequency offset to one of the beams so that zero object velocity will result in a non-zero frequency measurement. This solves the motion direction ambiguity (caused by the inability to differentiate between positive and negative frequencies) and it greatly increases the dynamic range (sensitivity to low velocities) by producing signals far from the DC components. The frequency offset also has other advantages related to signal identification and lock-on.
U.S. Pat. No. 5,587,785 to Kato, et. al. describes such a system. The frequency offset is implemented by providing a fast linear frequency sweep to the source beam before it is split. The method of splitting is such that a delay exists between the resulting beams. Since the frequency is swept, the delay results in a fixed frequency difference between the beams.
Multiple beams with different frequency offsets can be extracted by further splitting the source with additional delays. Each of these delays is then used for measuring a different velocity dynamic range.
A paper by Matsubara, et al., entitled "Simultaneous Measurement of the Velocity and the Displacement of the Moving Rough Surface by a Laser Doppler Velocimeter" (Applied Optics, 36, p. 4516, 1997) presents a mathematical analysis and simulation results of the measurement of the transverse velocity of a rough surface using an LDV. It is suggested that the displacement along the axial axis can be calculated from measurements performed simultaneously by two detectors at different distances from the surface.
In Homodyne/Heterodyne Doppler Measurements, a coherent light source is split into two beams. One beam (a "primary" beam) illuminates an object whose velocity is to be measured. The other beam (a "reference" beam) is reflected from a reference element, usually a mirror, which is part of the measurement system. The light reflected from the object and from the reference element are recombined (usually by the same beam splitter) and directed to a light-sensitive detector.
The frequency of the light reflected from the object is shifted due to the Doppler effect, in proportion to the object velocity component along the bisector between the primary beam and the reflected beam. Thus, if the reflected beam coincides with the primary beam, axial motion is detected.
The detector is sensitive to the light intensity, i.e.--to the square of the electric field. If the electric field received from the reference path on the detector is E.sub.0 (t)=E.sub.0 cos(.omega..sub.0 t+.phi..sub.0) and the electric field received from the object on the detector is E.sub.1 (t)=E.sub.1 cos(.omega..sub.1 t+.phi..sub.1), then the detector output signal is proportional to (E.sub.0 +E.sub.1).sup.2 =E.sub.0.sup.2 +E.sub.0 E.sub.1 +E.sub.1.sup.2.
The first term on the right side of the equation is averaged by the detector time-constant and results in a DC component The intensity of the reference beam is generally much stronger than that of the light reaching the detector from the object, so the last term can usually be neglected. Developing the middle term: ##EQU1##
From this equation it is evident that E.sub.0 E.sub.1 includes two oscillating terms. One of these terms oscillates at about twice the optical frequency, and is averaged to zero by the detector time-constant. The second term oscillates with frequency .omega..sub.0 -.omega..sub.1, i.e.--with the same frequency as the frequency shift due to the Doppler effect. Thus, the detector output signal contains an oscillating component with frequency indicative of the measured velocity.
It is common to add a frequency offset to the reference beam. When such a frequency bias is added, it is termed Heterodyne Detection.
U.S. Pat. No. 5,588,437 to Byrne, et al. describes a system in which a laser light source illuminates a biological tissue. Light reflected from the skin surface serves as a reference beam for homodyne detection of light that is reflected from blood flowing beneath the skin. Thus, the skin acts as a diffused beam splitter close to the measured object. An advantage of using the skin as a beam splitter is that the overall movement of the body does not effect the measurement. Only the relative velocity between the blood and the skin is measured.
The arrangement uses two pairs of detectors. Each pair of detectors is coupled to produce a difference signal. This serves to reduce the DC and low-frequency components interfering with the measurement. A beam scanning system enables mapping of the two-dimensional blood flow.
In Optical Coherence Tomography (OCT), a low-coherence light source ("white light") is directed and focused to a volume to be sampled. A portion of the light from the source is diverted to a reference path using a beam-splitter. The reference path optical length is controllable. Light reflected from the source and light from the reference path are recombined using a beam-splitter (conveniently the same one as used to split the source light). A light-sensitive detector measures the intensity of the recombined light. The source coherence length is very short, so only the light reflected from a small volume centered at the same optical distance from the source as that of the reference light coherently interferes with the reference light. Other reflections from the sample volume are not coherent with the reference light. The reference path length is changed in a linear manner (generally periodically, as in sawtooth waveform). This allows for a sampling of the material with depth. In addition a Doppler frequency shift is introduced to the measurement, allowing for a clear detection of the coherently-interfering volume return with a high dynamic range.
In conventional OCT, a depth profile of the reflection magnitude is acquired, giving a contrast image of the sampled volume. In more advanced OCT, frequency shifts, from the nominal Doppler frequency, are detected and are related to the magnitude and direction of relative velocity between the sampled volume (at the coherence range) and the measurement system.
U.S. Pat. No. 5,459,570 to Swanson, et al. describes a basic OCT system and numerous applications of the system.
A paper by Izatt et al., entitled "In Vivo Bidirectional Color Doppler Flow Imaging of Picoliter Blood Volumes Using Optical Coherence Tomography" (Optics Letters 22, p.1439, 1997) describes an optical-fiber-based OCT with a velocity mapping capability. An optical-fiber beam-splitter is used to separate the light paths before the reflection from the sample in the primary path and from the mirror in the reference path and combine the reflections in the opposite direction.
A paper by Suhara et al., entitled "Monolithic Integrated-Optic Position/Displacement Sensor Using Waveguide Gratings and QW-DFB Laser" (IEEE Photon. Technol. Lett. 7, p.1195, 1995) describes a monolithic, fully integrated interferometer, capable of measuring variations in the distance of a reflecting mirror from the measuring device. The device uses a reflecting diffraction element (focusing distributed Bragg reflector) in the light path from the source as a combined beam-splitter and local oscillator reflector. Direction detection is achieved by an arrangement that introduces a static phase shift between signals of the detectors.
Each of the above referenced patents, patent publications and references is incorporated herein by reference.